How does Descartes use god to avoid answering certain questions directly?
Following John Cottingham, I would suggest the answer has to do with what is called the "Cartesian Circle." While Descartes is often associated with radical doubt, the reality is that his Meditations escape from doubt pretty quickly. And the key to that is God.
Meditation 1 ends roughly at the idea that he could be deceived by an evil demon and everything is scrambled.
In Meditation 2, Descartes proceeds Ex hypothesi to argue that he exists -- but this is with the caveat that there is no evil demon. Here, he speaks of what is "clear and distinct" as that which is indubitable and that he (not as a man but as a thinking thing) exists as he has this feature.
In Meditation 3, Descartes presents a cosmological argument based on the existence of the idea of God to the existence of God insofar as he maintains that the only possible origin of a perfect and infinite idea must be perfect and infinite.
The thing is that the argument in Meditation 2 hinges on a guarantee that his mind is not being scrambled. But the argument in Meditation 3 hinges on him being able to use ideas he has in his minds. The mutual need for each proof is the "Cartersian Circle" and this makes it so he's not facing radical doubt by himself but rather one could say "using god to avoid" the effects of his initial radical doubt.
Before Meditation 4 he has already determined that God exists and that he is perfect.
In Meditation 4 he establishes that it is impossible that God is unfaithful to him, because God is perfect and being unfaithful is not perfect. Furthermore, he establishes that he has the ability to think, and that he has necessarily got the ability from God. Because God is perfect and thus faithful, he has not received the ability to think to mislead himself, as long as he uses the ability in the right way. Later in the Meditation, he tries to understand why he can make errors, and he concludes that it is because he has not only knowledge, but also the power of choice.
Descartes answer the question of the OP at the end of his 5th meditation (V.16):
And so I very clearly recognise that the certainty and truth of all knowledge depends alone on the knowledge of the true God, in so much that, before I knew Him, I could not have a perfect knowledge of any other thing. And now that I know Him I have the means of acquiring a perfect knowledge of an infinitude of things, not only of those which relate to God Himself and other intellectual matters, but also of those which pertain to corporeal nature in so far as it is the object of pure mathematics.
Before, Descartes had defined God - in accordance with the Scholastic tradition, the dominant philosophy and theology of his time (III.22):
By the name God I understand a substance that is infinite, independent, all-knowing, all-powerful, and by which I myself and everything else, if anything else does exist, have been created.
And in III.36 Descartes terminates his proof of the existence of God
[...] but we must of necessity conclude from the fact alone that I exist, or that the idea of a Being supremely perfect, i.e. that is the idea of God, is in me, that the proof of God's existence is grounded on the highest evidence.
Having obtained this result, Descartes in Meditation 5 draws his conclusions from the existence of God. Primarily, the existence of God ensures Descartes' truth-criterion and consequently justifies the whole epistemology of Descartes (V.15):
[I] have inferred that what I perceive clearly and distinctly cannot fail to be true.